Digital data networks are widely deployed throughout the United States. In such networks, a plurality of end user stations communicate among themselves or with a host station using a digital communication link called a local loop. The local loop is comprised of a four wire metallic line in which information is transmitted in one direction over two wires and received in the other direction over the two remaining wires.
At the network service office, which is typically the local telephone company, one end of the local loop is terminated at a Channel Bank. The Channel Bank contains a plurality of Office Channel Unit Data Ports (OCU-DP). At the customer's premises, the other end of the local loop is terminated at a Channel Service Unit (CSU) or Data Service Unit (DSU).
Many digital data networks employ a Network Interface Unit (NIU) to terminate the local loop on or near the customer's premises. The NIU is an electronic device provided by the local telephone company to establish a well defined demarcation point between the customer premises and the local loop. The NIU is instrumental in distinguishing between transmission problems on the local loop and transmission problems at the customer's premises. Telephone companies issue loopback commands to the NIU which program the NIU to enter into loopback mode. While in loopback mode, any data received by the NIU is returned to the sender. This allows the telephone company to perform Bit Error Rate Tests (BERTs) using the NIU as a loopback point and to determine if transmission problems are isolated to the local loop or include the customer's premises.
Without the NIU, the telephone company is limited to loopback points at the customer's Channel Service Unit or Data Service Unit (CSU/DSU). With only CSU/DSU loopback points available, the telephone company is unable to differentiate between local loop failures and customer premises failures. Using an NIU, the telephone company is able to differentiate between data transmission failures at the customer's premises and failures due to the local loop, resulting in more efficient resolution of transmission problems.